Process for the production of highgrade thomas steel



Dec. 9, 1958 J. KosMlDER 2,863,756

PROCESS FOR THE PRODUCTION OF HIGH-GRADE THOMAS STEEL Filed Oct. 22, 1956 v 2 Sheets-Sheet 1 Dec., 9, 1958 J, KosMlDER 2,863,756

PROCESS FOR THE PRODUCTION OF' HIGH-GRADE THOMAS STEEL lFiled oct. 22, 195e 2 sheets-sheet 2 .0&3

arent @bien $53,755 Patented Dec. 9, 1958 PRCESS FR THE PRDUCTION F HIGH- GRADE THMAS STEEL Johannes Kosmider, Hagen-Hasn, Westphalia, Germany, assigner to Huettenwerk Haspe A. G., Hagen-Haspe, Germany Application @ctober 22, 1956, Serial No. 617,418

7 Claims. (Cl. 7S-60) This application is a continuation-in-part of the copending application Serial No. 293,642, iiled June 14, 1952, now abandoned.

The present invention relates to a process for producing high-grade Thomas steel. More particularly, the invention relates to the production of high-grade Thomas steel of low nitrogen content not inferior in technical properties to Siemens-Martin steel and electro-steel.

Based on extensive research and experimental work the inventor has developed a process for the production of Thomas steel of similar content of phosphorus, nitrogen, and oxygen, and of the same technical properties as Siemens-Martin and electro-steel.

it is a known fact that Thomas steel made in the basic converted process is generally inferior to Siemens-Martin steel and electro-steel as far as workability in the cold is concerned.

Applicants research has shown that known operations carried with blast rich in ox gen (2S-92% O2) will not yield steel poor in nitrogen from pig iron of customary composition, i. e. containing about 3.5% C and 1-2% P, though cooling by scrap is adequate. In experiments with oxygen or oxygen-enriched blast gases, it was observed that increase in nitrogen in the period of de-phosphorization was the higher, the larger was the reduction of manganese from the slag in the transition.

According to studies made up to now, the increase in nitrogen content in the bath is `dependent on the partial pressure of nitrogen in the blast and waste gases, the bath temperature, and the blowing time. Low partial pressure of nitrogen in the Waste gases is important for decrease in nitrogen during the blowing period. The oxygen content of the blast gases decreases as they move upward through the bath, the consumption of oxygen starting at once upon the blast gases entering the bath. When the gases escape from the bath, they contain no free oxygen at all. In the decarbonization period, carbon is burnt partly to carbon monoxide, partly to carbon dioxide. The volume of the two gases formed in the reaction Aincreases during the reaction, as oxygen decreases, and the partial pressure of nitrogen is correspondingly decreased. If the partial pressure of nitrogen is not reduced, decrease in nitrogen may be prevented in the bath due to the high solution pressure of nitrogen: in some cases, there may even occur an undesirable increase of nitrogen in the bath. This is an important discovery since the nitrogen movement in the blast process is dependent on the partial pressure of nitrogen in both the blast and the waste gases.

Another characteristic for nitrogen increase in the bath in the de-phosphorization period is the manganese oxide reduction. A reduction from the slag can only take place if the bath is lacking in oxygen. Such lack in oxygen can not be met, inasmuch as increase in nitrogen is desired, by admitting pure oxygen to the furnace with the r blast gases, since this would cause the temperature to rise locally, even though scrap cooling takes place in a sucient degree: such local rise in temperature, however, automatically leads to increase in nitrogen.

It is the object of the present invention to overcome the above mentioned drawbacks.

According to the invention this can be achieved by blowing pig iron with low or normal contents in phosphorus and high contents in carbon (3.8-4%) with blast gases rich in oxygen, i. e. 30-5`5%, using a soda slag, a mixed soda-lime slag, or a lime slag, and adding ore 3-4 minutes before the transition, i. e. the end of the decarbonizing period. When pig iron of high carbon content and high or low phosphorus content is worked up in the converter in operating the process according to the invention, a high temperature and, consequently, an intense combustion of carbon is accomplished until shortly before the transition.

The partial pressure of nitrogen, which is reduced due to the increased oxygen content (of 30-55% is further decreased by formation of large amounts of carbon monoxide and dioxide, which leads to a considerable decrease of nitrogen in the iron bath.

By the use of soda or a mixture of lime and soda, a considerable de-phosphorization is already achieved in the de-carbonization period. In order to prevent the reduction of manganese at the beginning of the dephosphorization period, the necessary addition of ore occurs 3-4 minutes before the end of the decarbonizing period. The addition of ore prevents, at the same time, a harmful increase of the bath temperature, promotes the de-phosphorization, and decreases the combustion of iron. By the ore addition and the use of a soda or lime-soda slag, the de-phosphorization period is shortened by 30-50% as compared to the time of blowing required up to now for de-phosphorization. With the shortening of the blowing time, in spite of the excess of oxygen necessary at the beginning of the de-phosphorization, a steel low in nitrogen, phosphorus and oxygen is produced in the presence of a soda slag or lime-soda slag.

The inventor has found that it will be of advantage to add steam, or steam as well as carbon dioxide, 3 to 5 minutes before the end of the decarbonizing period.

According to an embodiment of the invention normal Thomas pig iron will be converted into high-grade Thomas steel by blowing the pig iron with a blast enriched with oxygen, introducing additional oxygen into the iron bath, de-phosphorizing the same, removing the slag and finally de-oxidizing the steel obtained after the removal of slag by means of steel iron. Steel iron or steel pig is a pig iron of characteristic analysis, namely about 3.8-4.2% C, 1.542.5% Mn, 0.61.2% Si, max. 0.4% P and max. 0.04% S. Applicant had found such steel pig to be an excellent means of cutting down excessive oxygen content of the melt at the end of the blowing process. The advantage of such pig is that this diminution of oxygen produces substantially gaseous deoxidation products, which therefore do not adversely a-ffeet the purity of the steel.

Example l A converter, having tuyere openings of 14 mm. each in the bottom, is charged with 20 tons of pig iron of the following composition:

and with 2500 kg. burned lime for slag formation. Through the holes in the bottom of the raised converter, a mixture of 35% O2 and 65% N2 is blown at a pressure of 2 atmosphere excess pressure. After Si and the major part of Mn have passed into the slag, de-carbonization sets in, noticeable by the brighter llame. In order to balance a deficiency in oxygen during the intense decarbomzation reaction, 1500 kg. of small-sized ore is added to the converter as a solid oxygen carrier after 5% minutes blowing time. The nitrogen contents of the melt has gone down to about 0.005% by this time. De-carbonization is completed after 9 minutes as may be observed from the diminishing flame; at this time cle-phosphorization begins to increase. During the de-phosphorization period increase in nitrogen contents will take place, which increases with a longer cle-phosphorization period. One of the advantages of the ore addition consists in the fact that it will result in a separation of phosphorus simultaneously while decarbonization takes place. The separation of phosphorus amounts to 4060%, While it is normally from 0-10%.

The time of the de-phosphorization period proper, which, among other factors, determines the contents of nitrogen, is considerably shortened. ln the present example, the dephosphorization period is 1.6 minutes, so that the entire blowing process will be 10.6 minutes. A steel of the following composition will be obtained before de-oxidation:

Percent C 0.05 Si traces Mn 0.12 P 0.05 S 0.025 N 0.0065

The temperature of the steel is 1630 C. Por the blasting process a total gas mixture of 3850 m was used comprising Tf1-3 O2 2500 111.3 N2 1500 kg. ore as solid oxygen carrier.

Example 2 The same converter is charged with tons of pig iron of the following composition:

Percent C 3.72 Si 0.40 Mn 0.90 P 1.8 S 0.55 N 0.007

after this total blowing time of 11.2 minutes the following analysis of steel is found:

Percent C 0.05 Si trace Mn 0.08 P 0.044 S 0.029 N 0.007

The steel temperature is 1635 C.

Example 3 ln the same converter, nearly the same pig iron as in Example 2 is blown with 35% oxygen enrichment. ln this case, however, the addition of 1500 kg. small lump ore is made after 2.75 minutes. The de-carbonization period ends after 10.2 minutes. Hence, the ore was added after 27% of preliminary blowing time, or 7.45 minutes before transition. The de-phosphorization period took 1.5 minutes, making a total blowing time of 11.7 minutes.

The final analysis of the steel is as follows:

Percent C 0.4 Si trace Mn 0.10 P 0.04 S 0.027 N 0.0095

The steel temperature is 1625 C.

It is apparent that the very early addition of orc, under otherwise like conditions, results in a higher final nitrogen content of the steel.

Example 4 In the same converter as in Example 2, using similar pig iron (only the Si content in this case being 0.27 22; a heat was blown with oxygen enrichment of 35% Og. ln this case, however, thc ore (1500 kg.) was not added until after 9.5 minutes. After 10.1 minutes, de-carbonization was finished. Thus, the ore was added alter 94% of preliminary blowing time, or 0.6 minute before transi- The steel temperature is 1635" C.

It is a known fact that despite decades of endeavors by those skilled in the art, the problem of producing Thomas steel has not been solved.

No dependable rules have been found for producing with accuracy Thomas steel of this kind prior to the teaching of the present inventor.

This teaching comprises the discovery of the optimum timing for the addition of ore. The addition of ore as such-or any sort of cooling in the converter processeshas long been known, of course; yet, it is not a mere difference in degree that in known processes, for example, a final nitrogen content in the steel of about 0.010%, and according to the method of this invention a nitrogen content of 0.005-0.006% is achieved. To obtain certain technological properties, even minute differences of 0.002-0.003% nitrogen are of substantial importance; for otherwise, decades of developmental work in Europe would not have been required for bringing the nitrogen content of Thomas steel close to that of Siemens-Martin steel. That determination of the optimum stage for addition of ore constitutes an invention, transcending the ability of the average person skilled in the art, appears from the graphs shown in the accompanying drawings wherein the movement of nitrogen in steel resulting from addition of ore is shown.

Fig. 1 shows the nitrogen movement generally during the blowing process in a converter handling Thomas pig iron. Curve 1 shows nitrogen content plotted against blowing time for an air-blown heat. The nitrogen content at the beginning of the blowing is generally between 0.006 and 0.007%, in the case of Thomas pig iron. During the first minutes of blowing there is a slight decrease in nitrogen content. With the de-carbonization period, however, the nitrogen content starts to rise again, and after removal of all carbon from the melt (transition), rises sharply to a final value of 0.0l5-0.020% in the steel. The sharpest rise of nitrogen in the de-phosphoriza tion period is indicated by the fact that the nitrogen partial pressure 1n the exhaust is l during this period.

Curves 2, 3, and 4 show the nitrogen movement during blowing with oxygen-enriched air. Since in this case the total blowing time is reduced, the graph does not state the blowing time numerically but in percent of total blowing time. Curve 2 shows the nitrogen movement for oxygen-enriched air when the ore is `added early in the process, after about 25% of the preliminary blowing period (period of blowing in the de-carbonization stage). Again, the nitrogen content rises gradually from the minimum midway through de-carbonization. This rise is less steep, and does not lead to such high final nitrogen content as in straight air heats; nevertheless, Values of 0.009% to 0.010% N in the final steel are reached. The situation is about the same when the ore is not added until late in the combustion of carbon, or in other words at the transition. The nitrogen content in the final steel is then between 0.010 and 0.012%. The lower nitrogen values compared to Thomas steel blown with straight air are due chiefly to the fact that oxygen enrichment causes somewhat more intensive denitrogenation of the molten pig iron by mid-decarbonization, and that the much abbreviated de-phosphorization period allows the steel less opportunity to re-absorb nitrogen. However, this method fails to depress nitrogen content below 0.008% in the final steel. Addition of ore as illustrated by curve 4, according to the instant application, is essential to ensure that a nitrogen content between 0.004 and 0.006% will be attained. These low nitrogen values are achieved by effecting the addition of ore at a time when certain processes are taking place in the converter. During the middle of the de-carbonization period, the rapidly burning carbon reduces iron oxide out of slag, this creating a reducing condition in the melt. This reducing condition, ,as shown, favors absorption of nitrogen. In this connection, reference may be made to the example of nitride hardening, where a reducing atmosphere permits migration of nitrogen into the steel, while oxidizing conditionsv prevent it. Because of these reducing conditions, the nitrogen content of the nal steel is above 0.008% in all methods, even with use of oxygen, unlessby addition of ore, las proposed in the present application-provision is made for a sufficiently abundant supply of oxygen during the main de-carbonization period to counterbalance such reducing conditions.

The implications of Fig. 1 are presented anew in Fig. 2, which shows how the final nitrogen content of the steel varies with the time of addition of ore. In the case of addition 3-4 minutes prior to transition, there is a definite minimum in the attainable nitrogen content of the final steel. Unavoidably, of course, the observed points scatter somewhat. This is indicated by the limiting curves plotted. For example, one can read off from Fig. 2 that when ore is added about 7 minutes before transition, nitrogen contents between 0.0085 and 0.010% are obtained in the final steel; or when ore is added 1 minute before transmission, nitrogen contents from 0.0095 to 0.011%.

While several preferred embodiments of the invention have been described heretofore, it should be understood that many modifications of the process described may be made without departing from the spirit of the invention.

What I claim is:

1. A process for the production of high-grade Thomas steel, which comprises blowing pig iron containing 1-2% of phosphorus and 3.8-4% of carbon with a blast of 30 to 55% of oxygen, wherein a slag is used selected from the group consisting of soda, lime-soda, and lime; and adding ore 3-4 minutes before the end of the decarbonizing period.

2. A process according to claim 1, wherein 3-5 minutes before the end of the decarbonizing period steam is added.

3. A process according to claim 2, wherein 345 minutes before the end of the decarbonizing period carbondioxide is added as well.

4. A process according to claim 1, wherein the oxygen amount introduced into the iron bath is adjusted in accordance with the desired nal nitrogen content of the steel.

5. A process for the production of high-grade Thomas steel, which comprises blowing pig iron containing 1-2% of phosphorus and 3.8-4% of carbon with a blast of 30 to 55% of oxygen, and using as a slag one selected from the group of soda, lime-soda, and lime, and introducing gaseous oxygen into the iron bath 3-5 minutes before the end of the decarbonizing period.

6. A process according to claim 5, wherein the additional oxygen amount introduced into the iron bath is adjusted in accordance with the desired final nitrogen content of the steel.

7. A process for the production of high-grade Thomas steel, which comprises blowing Thomas pig iron with a blast enriched with oxygen, introducing additional oxygen, de-phosphorizing and removing the slag, and finally cie-oxidizing the steel so obtained by means of pig iron consisting of about 3.8-4.2% C, 1.5-2.5% Mn, 0.6-1.2% Si, max. 0.4% P and max. 0.04% S.

References Cited in the file of this patent UNITED STATES PATENTS 217,495 Thomas July 15, 1879 253,130 Thomas Ian. 31, 1882 1,032,653 Brassert July 16, 1912 2,244,385 Brassert June 3, 1941 2,466,163 Ekman et al Apr. 5, 1949 2,598,393 Kalling et al. May 27, 1952 2,671,018 Graef Mar. 2, 1954 2,707,677 Graef May 3, 1955 FOREIGN PATENTS 16,440 Great Britain May 23, 1907 of 1906 713,300 Great Britain Aug. 11, 1954 717,975 Great Britain Nov. 3, 1954 OTHER REFERENCES Transactions A. I. M. E., vol. 145, pages 126, 128, 142, 145, 149, 156, and 157; publ. by the A. I. M. E., New York, N. Y. (1941). 

1. A PROCESS FOR THE PRODUCTION OF HIGH-GRADE THOMAS STEEL, WHICH COMPRISES BLOWING PIG IRON CONTAINING 1-2% OF PHOSHPORUS AND 3.8-4% OF CARBON WITH A BLAST OF 30 TO 55% OF OXYGEN, WHEREIN A SLAG IS USED SELECTED FROM THE GROUP CONSISTING OF SODA, LIME-SODA, AND LIME; 